Covalent azide

Reactions of ionic or covalent azides with chalcogen halides or, in the case of sulfur, with the elemental chalcogen provide an alternative route to certain chalcogen-nitrogen compounds. Eor example, the reaction of sodium azide with cyclo-Sa in hexamethylphosphoric triamide is a more convenient synthesis of S7NH than the S2CI2 reaction (Section 6.2.1). Moreover, the azide route can be used for the preparation of 50% N-enriched S7NH. 

The third class of compounds to be discussed in this chapter are those in which an RE group (E = S, Se, Te) is attached to a nitrogen centre. This category includes amines of the type (REfsN and the related radicals [(RE)2N] , as well as organochalcogen(ir) azides, REN3, and nitrenes REN (E = S, Se). Covalent azides of the type RTe(N3)3 and R2Te(N3)2, in which the chalcogen is in the +4 oxidation state, have also been characterized. 

A remarkable new covalent azide is the pale yellow nitrosyl NNNNO, prepared by reacting gaseous NOCl (p. 441) with solid NaN3 at low temperature.NNNN(S02p)2 has also very recently been made by a similar route from (S02F)2NC1 it is a volatile yellow liquid which sometimes decomposes explosively. 

The difference in stability of ionic and covalent azides is sometimes explained in terms of resonance structures. The azide ion, N3 can be represented by the three resonance structures... 

Structure I is the most important of the three. A covalent azide such as HN3 (dipole moment = 1.70 D) can be represented by the resonance structures... 

A series of 12 complexes of Co(III) with both ionic and covalent azide groups was prepared and most were easily detonable as dry salts, especially at elevated temperatures [1]. Polarography is an accurate and safe method of analysis for azides [2], Hexaammine-cobalt, -chromium and -rhodium hexaazidocobaltates are explosive, particularly in the dry state [3],... 

Alkynes have been well explored as dipolarophiles in the [3 -t- 21-cycloaddition with almost all possible 1,3-dipoles (78), whereas the reaction of iminoboranes as dipolarophiles has focused on covalent azides as 1,3-dipoles. Most well-characterized iminoboranes were reacted with phenyl azide, according to Eq. (52) (11-14,17, 20). 

The reactions of covalent azides with —C=N— systems have been discussed <77AHC(21)323,... 

Silylene-azide complexes are probable intermediates in the formation of silaimines from silylenes and covalent azides (equation 66)147,148. 

In a number of ways the reactions of stable silylenes resemble those of phosphines, R3P, to which they are isolobal analogs. Examples are provided by the reactions of 59 with covalent azides. Phosphines are known to react with azides to give phosphineimines, Ph3P=NR. In similar fashion, 59 reacted with triphenylmethyl azide in THF to give the silanimine 72 as its THF complex (equation 109)148. This reaction provides a new method for synthesizing compounds containing Si=N double bonds, which have previously been made by salt elimination reactions375. 

IONIC AND COVALENT AZIDES THE ADJACENT CHARGE RULE... 

Form C for the acid, however, contains two adjacent atoms having positive formal charges whereas form C for the anion does not. If we consider structures containing adjacent atoms of like charge as inadmissible, then covalent azides should have but two forms as compared with three for ionic azides. While such a correlation may be far fetched, azide behavior is predicted correctly. Azides of the alkali and alkaline-earth metals (ionic azides) decompose at much higher temperatures than do azides of the heavy metals or hydrazoie acid itself (which are predominately covalent). Similarly, simple organic azides such as methyl azide, CH3N3, for which only two admissible forms may be drawn tend to be less stable than acid... 

All of these structures contribute to the true structure although structure I is certainly the dominant one. However, a covalent azide such as HN3 represents a somewhat different situation as shown by the following structures ... 

For references see T. M. Klapotke et aL, Inorg. Chem. 1995,34, 4343 I. C. Tornieporth-Oetting and T. M. Klapotke, Angew. Chem. bit. Ed. Engl. 1995,34, 511 (covalent azides 78 references and much data). 

In covalent azides, the pseudohalogen azide RN3 has an angular structure as in HN3. Triazidoborazine [H3N3B3(N3)3] and other boron azides, for example, salts of the tetraazidoborate ion B(N3 (4 have been considered as boron nitride precursors (see Boron-Nitrogen Compounds). The M(Ns)3 azido complexes of the other group 13 elements (Al, Ga, In, Tl) and their M(N3)4 tetraazido anions are all known. They and their derivatives are also used as precursors for the nitrides. 

Covalent azides add to alkenes in a [2 + 3] cycloaddition, giving triazolines (Volume 4, Chapter 4.10), and these in turn can be thermolyzed or photolyzed with loss of nitrogen to give aziridines. The addition obeys the complementarity principle for cycloadditions, which in this case means that electron-rich azides prefer to react with electron-poor alkenes and vice versa. Thus, a few N-alkylaziiidines have been prepared from electron-poor alkenes via triazolines. The thermal stability of triazolines with respect to loss of nitrogen and aziridine formation varies with the electronegativity of the exocyclic N-sub-stituent. Thus, V-vinyltriazolines decompose more readily than N-alkyltriazolines to give V-vinylaziridines in good yield, as in Scheme 18. Several examples of intramolecular formation of... 

The reactions of covalent azides with the isocyanate system are summarized in Scheme 41. Addition of organic azides and azide ion to isocyanates occurs at the —N=C— bond and gives 1,4-disubstituted tetrazolin-5-ones (221 Scheme 41). The reaction of organic azides with isothiocyanates is a complicated reaction giving a mixture of five different products of which the tetrazoles (223) are minor constituents and the remaining compounds are sulfur heterocycles. Organometallic azides, and also azide ion, add to the —N=C—... 

The general shape oi covalent azides is shown in Figure 1. 

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